Nine patients with history of hydatid disease have been examined by CT. Localization of the hydatid cysts in the pelvis was established by anatomical criteria. Occasionally, the transverse plane can be confusing for the precise localization of a lesion. A central location, close to the boundaries of the bladder and rectum, can define peritoneal location. Further posterolateral retrovesical location can be considered retroperitoneal. Using these criteria, 8 cysts were situated within the peritoneum and 1 within the retroperitoneum.